Image pickup tube

ABSTRACT

An evaporator for an image pickup tube comprises base metal sources attached to symmetrically disposed bent portions of otherwise straight heater lines wherein the radius of a circle connecting these sources is greater than in conventional arrangements of this type.

United States Patent [72] Inventor Yorikatsu lnsak' a [56] References Cited Yokoha a-s Japan UNlTED STATES PATENTS [21] P 2,189,322 2/1940 Flory 313/s1x [22] F1led June 12, 1968 FOREIGN PATENTS [45] Patented Feb. 16, 1971 [73 1 Assignee Tokyo s m Ekctric 3 1,, 552,868 5/1957 Belgium 313/176 xawasiki'shis l Primary Examiner- Rodney D. Bennett, Jr. Pnomy June 151 1967 Assistant Examiner-Joseph G. Baxter Japan Attorney-Stephen H. Frishauf [31] 42/347842 [54] IMAGE PICKUP TUBE 7 Claims, 6 Drawing Figs.

[52] US. 313/180, ABSTRACT: An evaporator for an image pickup tube com- 313/176 prises base metal sources attached to symmetrically disposed [51] Int- H 0lj 19/68 bent portions of otherwise straight heater lines wherein the [50] Field of Search 313/178 radius of a circle connecting these sources is greater than in 181, 180, 176, 174(issue); 315/10, 11'

conventional arrangements of this type.

PATENTEU FEB] s 1971 SHEET 1 HF WYRIKRTW 1mm,

PATEN-TED FEB] 6 WI SHEET 2 BF 4 YORIKATSU IRISAKA PATENTEUFEBI s 1971 sum 9 or 4 INVENTOR.

YORIKATSU lR-ISAKA IMAGE PICKUP-TUBE The present invention relates to an image pickup tube evaporator having an improved evaporator for theformation of the photoelectric surface. Y

A prior art electron tube having a photoelectric surface, such as an image pickup tube, has a sealed cylindricalenvelope provided with an enlarged portion at one end in which is disposed a plurality of electrodes. e

The end of the enlarged portion ofthe envelope is provided with a faceplate covered at its peripheral margins with a conductive metal-film serving as the conductive terminal for a photoconductive surface. The electrodes are assembled into a unit by means of a ceramic material in ,parallel with each other and to the face plate. The evaporator, which is provided with an evaporating metal such as an alloy of silver and bismuth, is mounted within a flanged portion of one of the electrodes. The evaporator usually consists of at least one thin metal wire substantially in the form of a circle which is provided with one or more spot sources of base metal to be evaporated.

As is well known in the art, it is desirous for the photoelectric surface to be uniformly and highly sensitive over its-entire service area, the sensitivity being highly dependent upon the thickness of the photoelectric layer which is most preferably so thin that it shows no conductivity by itself and becomes conductive only when an alkali metal is deposited on it. To meet this end, the old so-called single point evaporator has been replaced by the multipoint evaporator in which the evaporating substance is deposited atsymmetrical points on a heater wire curved substantially along the curvature of the side face of the tube. The electrode assembly is. disposed within the enlarged portion of the tube envelope and comprises an accelerating electrode for accelerating electrons given off at from photoelectric surface, a decelerating elec' trode for decelerating electronsfro'm the electron gun and a target electrode for accumulating decelerated electrons. However dimensional restrictions are imposed on the electrode assembly on account of electric field lenses to be established by individual electrodes. As a consequence, restrictions are also imposed upon the distance between the evaporator and the faceplate and upon the diameter of the evaporatoL Due to the restrictions upon the position of the evaporator, the thickness of the photoelectric layer obtained lacks uniformity, resulting in fluctuations in .the sensitivity of the photoelectric surface. As it is necessary'that the photoelectric layer be so thin that it is practically nonconductive 'with the base metal alone and becomes conductive only whenv the alkali metal. is subsequently deposited thereon, the photoelectric layer formed by the evaporator of the usual image orthicon has a thicker central region and a thinner peripheral region. Asthe result when the thickness of the central portion is made standard, the peripheral portions are too thin to have sufficient conductivity, causing a potential gradient which gives rise to distortion of the image.

The principle object of this-invention is to eliminate the aforementioned deficiencies by the provision of an image pickup tube having a uniformly thick photoelectric layer which is highly sensitive and has no sensitivity fluctuations. An investigation has been conducted with respect to the uniformity of the photoelectric layers. Three factors were varied: the diameter of the photoelectric layer, the distance between the evaporator and the photoelectric surface and the diameter of a circle connecting the point sources of the multisource evaporator. it has been confirmed that it is necessary to increase the diameter of the circle connecting the point sources of the evaporator in a conventional image orthicon to meet the object of this invention.

SUMMARY or THE INVENTION trons emitted from the photoelectric layer, an electron gun shooting an electron beam at the target, means for scanning the target by the'electron beam, means for for deriving video signals in correspondence with charges accumulated on the target, and a base metal evaporator interposed between said target and the faceplate, the evaporator comprising heating lineshaving a plurality of bends arranged symmetrically such that the bends define the outermost contour of said heater lines the plane defined by said bends being parallel with said faceplate, base metal being provided on said bends.

IN THE DRAWINGS FIG. 1 is a simplified schematic longitudinal sectional view of one embodiment of the image orthicon according to the invention;

FIG. 2 illustrates the manner of determining the thickness distribution of the thin layer of base material formed when the distance between the four point source evaporator and the photoelectric surface, hereinafter referred to as the relative height, is 1.3 units where one unit is the radius of the circle connecting the point sources of the four point source evaporator of the image orthicon; 7

FIGS. 3 and 4 illustrate the thickness distributions of the photoelectric layers with the relative heights of 1.1 and 1.44 units respectively; i FlG. 5 is a transverse section, to an enlarged scale, of an evaporator of the image pickup tube embodying the invention; and FIG. 6 is a more detailed, partial blowup of the portion of a tube similar to that of FIG. 1 to the upper right-hand side of the image orthicon showing the cooperation between elements in more detail.

Referring to the drawings, and particularly to FIG. 1, the image orthicon to which the present invention applies consists of three sections, namely, an image section 10, a scanning section 30' and an electron multiplier section 50, these sections being enclosed within an evacuated envelope 1.

Light incident on,a face plate 17 through optical lenses not shown produces photoelectrons 3 in an amount proportional to the intensity of the incident light 2 at a photoelectric surface 11 of the faceplate. The photoelectrons thus generated are accelerated by an accelerating electrode 12 in the direction parallelto the axis of the tube towards the targetmesh assembly 14 secured on a target-mesh assembly cup 13. The target 14 consists of a mesh 15 and a target glass 16. The surface of the target glass 16 is' coated with a secondary electron emissive material such as cesium. When photoelectrons accelerated through the accelerating electrode 12 and having cleared mesh 15 impinge upon the target glass 16, secondary electrons are given off at the surface of the target plate which are caught by the mesh 15. Accordingly, those portions of the target area which have received the greater amounts of light radiation provide correspondingly greater amounts of photoelectrons in proportion to which the secondary electrons are given off at the target glass. Thus, greater or less amounts of light incident upon the photoelectric surface is represented by correspondingly greater or less amounts of positive electric charge accumulated in the target glass. 7 The image formed of positive charges accumulated in the target glass is scanned by the scanning section 30 to take out (or derive) the corresponding video signal current. The electron beam provided by the cathode 32 of an electron gun 31 is accelerated by electrodes 34, 35, 36 and '37 toward the targetmesh assembly 14. Between the cathode and the target, there is provided an aligning coil 39, a focusing coil 40 and a horizontal and vertical deflection coil 41 for the control of an electron beam 33. The electron beam approaching the targetmesh assembly 14 is decelerated by a decelerating electrode 38 for the low-speed beam scanning. Part of the electron beam scanning the target glass neutralizes with the positive charges representing the image, with the rest of the beam returning to the electron gun.

Around the electron gun 31 is disposed the secondary electron multiplier section 50. When the returning beam strikes the first dynode 51, whose surface is coated with beryllium oxide or chromium oxide, many secondary electrons are given off, which are multiplied by the dynodes 52, 53, 54 and 55 of the secondary electron multiplying section. The multiplied electrons are caught by a plate 56 to be converted into a video signal. The dynodes 52, 53, 54 and 55 are made of, for instance, an alloy of silver and magnesium and their surfaces are treated with magnesium oxide.

The image section is now described in detail to assist the understanding of the entire specification. The peripheral margin of the faceplate 17 of the evacuated envelope 1 is coated with the conductive metal film 18 serving as the terminal for the photoelectric layer 11 to be deposited thereon. The target cup 13 is provided with an annular groove-shaped flange 19 open toward the faceplate. In the groove-shaped flange 19 is disposed an evaporator 20. The evaporator 20 consists of a platinum-clad molybdenum wire, which is capable of heating by electric current on which is disposed base metal for the photoelectric layer, such as an alloy of silver and bismuth, antimony and the like at symmetrical points on the wire. Outside a gap which is formed between the accelerating electrode 12 and the target cup 13 is provided an alkali metal evaporator 24 for evaporating an alkali metal such as cesium and depositing it on the base metal of the photoelectric layer 11 for the purpose of improving the sensitivity thereof. To improve the sensitivity a study has been conducted about the position of the evaporator relative to the photoelectric layer. It has been found that in an actual image orthicon the radius of the multipoint source evaporator should be made as great as possible. The invention, accordingly, proposes a way of positioning the evaporator within the image pickup tube.

In the evaporative deposition of base metal from the evaporator consisting of a plurality of droplets which are regarded practically as points of metal to be evaporated on a thin metal wire, only that part of the evaporated metal gas is utilized which proceeds from the evaporator in the direction substantially normal to the plane defined by said thin wire.

Denoting a point on the plane of the faceplate at which the angle defined by this plate and the line connecting that point and a point metal source is by P0, the thickness T6 of the evaporated film at point P6 is expressed by the equation where ()is an angle between the faceplate and the line connecting each of the point sources at that point.

The contour lines representing the layer thickness as depicted by using the equation 1 are apparently concentric circles. The contour lines for a two point source evaporator is synthesized from contour lines for individual point sources. The contour lines for a four point source evaporator may also be determined in the same way. FIG. 2 illustrates the way of synthesizing the individual groups of contour lines and the synthesized final contour lines representing the layer thickness distribution of a planar photoconductive layer on the faceplate obtained by a four point source evaporator with their point sources lying on a circle of one unit and parallel to the faceplate and having a relative height of 1.3 units. The numerical figures assigned to the individual contour lines indicate the thickness of the deposited layer by corresponding units when the layer thickness obtained by a single source at a point directly beneath the source is one unit. As is apparent from FIG. 2, the obtained contour lines are substantially define a square, the central portion being approximately plane and uniform and thinning out toward the end. In FIGS. 2,3 and 4, the points directly beneath the point sources are represented by crosses. FIGS. 3 and 4 illustrate similar contour lined depicted in the same manner as for FIG. 1, with FIG. 3 representing the thickness distribution for the relative height of 1.1 units while FIG. 4 for the relative height of 1.44 units. This mode of depicting the contour lines may be used for evaporators having eight symmetrically disposed point sources as well. In this case the contour lines are substantially concentric circles and the thickness of the layer is outstandingly uniform, the uniformity being most pronounced when the relative height is one unit.

Table 1 lists maximum and minimum values of layer thickness of the deposited layers having various areas equal to the circles connecting four point sources, that is relative heights.

TABLE 1 Relative height 1.0 1.1 1.2 1.3 1.4

Minimum thickness. 1. 376 1. 522 1. 661 1. 792 Maximum thickness. 1. 542 1. 661 1. 816 1. 992 2. 156 Ratio of minimum to maximum.. 0.800 0. 829 0. 839 0. 834 0. 832

TABLE 2 3-Inch Image Orthicon Relative height 1.0 1.1 1.2 1.3 1.4 1. 5

Evaporator diameter D in mm- 72 65. 4 60. 0 55. 4 51. 4 48. 0 55 .764 0. 842 0. 917 0. 993 1. 07 1. 145 Minimum thickness- 1. 485 1. 59 1. 67 1. 725 1. 78 Maximum thickness 1. 661 1 816 1. 992 2. 156 2. 305 Ratio of minimum to ma mum 0.89 0. 89 0. 87 0. 84 0.80 0. 774

Z-Inch Image Orthicon Relative height 1.7 1.9 2.2

Evaporator diameter D in mm 41. 2 36. 84 31. 8 55 0. 90 1. 006 1. 192 Minimum thickness- 2. 228 2. 38 2. 408 Maximum thickness 2. 56 2. 768 3. 048 Ratio of minimum to maximum 0. 863 0. 841 0. 79

As is apparent from table 2, with the 3inch image orthicon the layer thickness is most uniform when the relative height is in a range of 1.0 to 1.1, the uniformity becoming less when the relative height is increased. The minimum relative height available with the presently used aforementioned electrode is 1.22, the maximum being of the order of 1.72, so that it is impossible to have a relative height of 1.1. The 2-inch image orthicon is better than the 3-inch image orthicon in view of the uniformity of the photoelectric surface layer but is by no means best.

Accordingly, with imagetubes having restrictions upon the radius of the evaporator circle and the distance between the evaporator and the face plate, the photoelectric layer has thicker central portion and thin portions at the peripheral end, the thickness at the end being about 79 percent of the central portion.

Consequently, the sensitivity is irregular and in some instances the layer thickness near the peripheral end is to small to provide conductivity, thereby resulting in establishment of a potential gradient in the radial direction across the photoelectric layer and undesired distortion of the image.

According to the invention the above-described disadvantages are overcome by the provision of an image pickup tube in which the diameter of the evaporator circle is substantially increased by constructing it with 'metal wires having symmetrically disposed bends on which is deposited the base 5 metal to be evaporated. 7

Referring now to FIG. 5,- by reference character 20 is generally designated a preferred embodiment of the evaporator according to the invention mounted in an annular grooveshaped flange 19 of the target cup. electrode of FIG. 1. The evaporator assembly 20 comprises four-substantially spherical droplets about 0.5 mm. in diameter of base metal to be evaporated and deposited to on the faceplate to form a photoelectric layer. The evaporator includes droplets of base metal such as an alloy of silver and bismuth, deposited on four respectiveplatinum-clad molybdenum wires 21 of about 0.2 mm. in diameter symmetrically disposed and having bends 22a ,22b, 22c and 22d. The bends define the outermost contour of the evaporator. The ends of the straight portions of the four individual wires 21"are secured to respective supports 23a, 23b, 23c and 23d, by means of, for instance, electrical soldering, so as to form a closed more or less polygonal loop whose plane is parallel to the plane of the faceplate. The support 2 3a, 23b, 23c and 23d also terminals for wires21 through 25 which electric current is caused to evaporate the base metal. The evaporator just described may be placed very close to the target electrode The outermost contour of the evaporator of FIG. 5 is a circle connecting the four point sources (at bends 2211-22, respectively) has a greater diameter than in the conventional evaporator. The ratio of minimum to maximum layer thicknesses is improved'from 0.80 for conventional products to 0.87 thus decreasing the nonuniformity of the layer thickness and decreasing the irregularity of the sensitivity in the service area of the photoelectric surface. As the sup- 3 5 ports 23a-23d are located as far as possible from the bends 22042, geometrical arrangement of the point sources of a two point source modification of the evaporator is restricted to an ellipse, on focus points of which lie two supports, and on the intersections of the ellipse with the minor axis lie two point 40 sources. As the elongation of the straight portion of the wires 21 when heated by electric current is extremely small, the evaporator may be disposed in close proximity to the inner periphery of the target electrode without the possibility of the bend of the wires 21 touching the target electrodes at the time of heating. Further, the existence of bend is extremely convenient for the deposition of the base metal on the wire 2.1. It is easier to deposit the base metal with this configuration. Also, the operating steps are minimized as compared to the conventional evaporator. 4

The invention has been described with respect to an image orthicon tube but the invention may also be applied to the usual image tubes.

The present invention, accordingly, provides simple construction, high yield and improved performance.

While the invention has been described in connection with some preferred embodiments thereof, the invention is not limited thereto and includes any modifications and alternations which fall within the scope of the invention.

I claim:

1. An image pickup tube including:

an evacuated cylindrical envelope having a faceplate at one end thereof;

a photoelectric layer formed on said faceplate;

a target provided opposite said photoelectric layer, said target having a central portion;

an accelerating electrode for accelerating photoelectrons emitted from said photoelectric layer;

an electron gun directing an electron beam at said target;

means for scanning said target byv said electron beam;

means for deriving video signals in correspondence with electric charges accumulated on said target;

the improvement wherein; a base metal evaporator IS interposed between said target and said face plate, said base metal evaporator compris- A heating line having a plurality of symmetrically arranged bends therein, said bends lying in a plane which is substantially parallel with said face plate, and said bends defining the outermost contour of said heating line in said plane within said tube, said contour being outermost relative to an axis extending perpendicular to said target and through said central portion of said target; and

base metal provided on said line at said bends.

2. An image pickup tube as claimed in claim 1 in which said line has four bends therein, base metal being provided on said four bends.

3. An image pickup tube as claimed in claim 1 in which said heating line includes four metal wires, each having a bend therein, connected in an annular form by support means.

4. An image pickup tube as claimed in claim 3 comprising fourmetal support means connecting said wires in said annular form, said support means being electrically connected to said wires to serve as electrical terminals for said heating line.

5. An image pickup tube as claimed in claim 1 wherein said outermost contour is a circle connecting said bends.

6. An image pickup tube as claimed in claim 1 which said base metal is an alloy of silver and bismuth.

7. An image pickup tube as claimed in claim 1 in which said 0 base metal is antimony. 

1. An image pickup tube including: an evacuated cylindrical envelope having a faceplate at one end thereof; a photoelectric layer formed on said faceplate; a target provided opposite said photoelectric layer, said target having a central portion; an accelerating electrode for accelerating photoelectrons emitted from said photoelectric layer; an electron gun directing an electron beam at said target; means for scanning said target by said electron beam; means for deriving video signals in correspondence with electric charges accumulated on said target; the improvement wherein: a base metal evaporator is interposed bEtween said target and said face plate, said base metal evaporator comprising: A heating line having a plurality of symmetrically arranged bends therein, said bends lying in a plane which is substantially parallel with said face plate, and said bends defining the outermost contour of said heating line in said plane within said tube, said contour being outermost relative to an axis extending perpendicular to said target and through said central portion of said target; and base metal provided on said line at said bends.
 2. An image pickup tube as claimed in claim 1 in which said line has four bends therein, base metal being provided on said four bends.
 3. An image pickup tube as claimed in claim 1 in which said heating line includes four metal wires, each having a bend therein, connected in an annular form by support means.
 4. An image pickup tube as claimed in claim 3 comprising four metal support means connecting said wires in said annular form, said support means being electrically connected to said wires to serve as electrical terminals for said heating line.
 5. An image pickup tube as claimed in claim 1 wherein said outermost contour is a circle connecting said bends.
 6. An image pickup tube as claimed in claim 1 which said base metal is an alloy of silver and bismuth.
 7. An image pickup tube as claimed in claim 1 in which said base metal is antimony. 